The first shots came before sunrise. At 4:45 AM on September 1, the German pre-dreadnought battleship *Schleswig-Holstein* opened fire on the Polish military transit depot at Westerplatte in the Free City of Danzig. Simultaneously, 1.5 million German troops, supported by 2,000 aircraft and 2,500 tanks, surged across the Polish frontier along three fronts. The Slovak Republic, a German client state, fielded the Bernolák Army with over 50,000 soldiers to join the invasion from the south. The Polish army, outmatched in mobility and firepower, fought delaying actions from the start.
This was not a spontaneous act. The staged Gleiwitz incident the previous night, where German operatives dressed in Polish uniforms attacked a radio station, provided a thin pretext. Adolf Hitler’s directive stated the political aim was not the mere restoration of Danzig but "the destruction of Poland." The strategy was Blitzkrieg—concentrated armor and air power punching holes in enemy lines, bypassing strongpoints, and disrupting command. The Luftwaffe gained air superiority within 48 hours, bombing cities and strafing columns of refugees to sow terror.
The invasion triggered formal declarations of war from Britain and France on September 3, but it produced little immediate military relief for Poland. The Soviet Union invaded from the east on September 17, sealing the country's fate under the secret protocol of the Molotov-Ribbentrop Pact. Organized Polish resistance collapsed by October 6. The campaign established a brutal template. SS Einsatzgruppen followed the Wehrmacht, beginning systematic executions of Polish intellectuals, officials, and Jews. Germany and the USSR divided the country, commencing an occupation of unparalleled savagery.
September 1, 1939, is conventionally marked as the start of World War II in Europe. It ended two decades of an uneasy peace and began nearly six years of total war. The invasion demonstrated the horrifying efficiency of combined arms warfare and ideological ruthlessness. It collapsed the international order established in 1919 and initiated a conflict that would redraw the map of the continent and define the remainder of the century.